Figure 2. XRD image of I4/mcm MgO₂ powder synthesized at 134 GPa (seen as dark grey vertical lines) in rectangular coordinates (cake).

Red and violet ticks correspond to the positions of I4/mcm MgO₂ and MgO, respectively. Green ticks represent some reflections of ζ-O₂ (high angle Bragg reflections are not shown). White labels are Miller indices of the indexed tetragonal phase. Part of this XRD pattern (2θ = 9–13.5) was used to Rietveld refine the predicted structure of I4/mcm MgO₂. The x-ray wavelength is 0.3344 Å.